Probably because it is built on tools that are made to scale arbitrarily and solve a large amount of very general needs. This need is simple, the tool is not built for simple needs though it can meet them.

I dislike the UX of AWS but the complexity here isn't strange to me. I bothered to set up the first bit of this (didn't need sub-dir index files so didn't realize it was a problem) since I wanted a simple storage, simple deploy and good resilience.

My site has been hit by HN a few times now and it hasn't been an issue. Fairly set and forget. But the start is annoying.